The apparatus for measuring the shape of corneal surface is an ophthalmic apparatus which is useful for prescribing contact lens or for examining the corneal shape before and after an LASIK(Laser Associated Stromal In situ Keratomileusis) operation. The apparatus for measuring the shape of the corneal surface is generally called as a corneal topographer. Recently developed corneal topographer can also obtain the thickness of the cornea by measuring the shape of the endothelial surface and the shape of the epithelial surface thereof.
A prior topographer radiates an LED light of an infrared wavelength onto a cornea from a single ring shaped light source, detects the ring image reflected from the cornea with a two-dimensional CCD (Charge Coupled Device), and obtains the curvature of the cornea by analyzing the reflected ring image. In an improved topographer, the light source is designed to include a plurality of illumination rings, and thereby one can obtain the two-dimensional curvature distribution of the cornea by analyzing the ring images reflected from the cornea. Meanwhile, a multi-functional ophthalmic apparatus for measuring the shape of the corneal surface and the refractive power of an eye at the same time is also developed.
FIG. 1 is a drawing to illustrate the optical system of the conventional corneal topographer. As shown in FIG. 1, the conventional corneal topographer comprises a chart image generating part 20 for projecting an image to fixate the gaze of the examined eye 1; a collimating light source 30 for projecting a collimating light to fixate the position of the eye 1 with respect to the topographer; a measuring light source 32 for projecting concentric ring-shaped measuring lights to the cornea 2 of the eye 1; and a light detector 40 for detecting the images of the collimating light and the measuring lights reflected from the cornea 2 of the eye 1. The chart image generating part 20 comprises a lamp 22 for radiating a visible light; and a chart 24 for passing the light produced by the lamp 22, and generating a chart image to be projected to the eye 1.
In operation, the visible light radiated from the lamp 22 of the chart image generating part 20 passes through the chart 24, and forms a chart image. The image formed at the chart 24 is projected to the retina 3 of the eye 1 through the first relay lens 25, a reflecting mirror 26, the second and third relay lens 27,28, the first and the second beam splitter mirror 35,36, and the cornea 2. The patient fixates his or her gaze at the produced chart image, which prevents the measurement error of the corneal shape due to the movement of the eye 1 to be examined.
When the gaze of the eye 1 is fixated, the collimating light is radiated from the collimating light source 30. The collimating light passes through an objective lens 34 and the first beam splitter mirror 35 and forms an image of the collimating light at the cornea 2. The image of the collimating light reflected from the cornea 2 is partially reflected by the first beam splitter mirror 35, and then the partially reflected image passes through the third relay lens 28, the second beam splitter mirror 36 and the fourth relay lens 37, and forms the reflected image of the collimating light on the light detector 40. The operator moves the position of the topographer to most clearly detect the image of the collimating light formed with the light detector 40, so that the position of the eye to be examined is properly fixated with respect to the topographer.
When the gaze and the position of the eye 1 are fixated, a plurality of the concentric ring-shaped measuring lights is radiated to the cornea 2 of the eye 1 from the measuring light source 32. The measuring lights reflected from the cornea 2 pass through the first beam splitter mirror 35, the third relay lens 28, the second beam splitter mirror 36, and the fourth relay lens 37, and form the ring-shaped images of the measuring lights reflected by the cornea 2 on the light detector 40. The shapes of the measuring light images formed on the light detector 40 changes according to the corneal shape of the eye 1. Thus, the shape of the corneal surface and the curvature of the cornea can be calculated and obtained by analyzing the images formed on the light detector 40 with a conventional processor installed in the topographer. In summary, the conventional topographer radiates ring images from the measuring light source 32 consisting of LEDs of the Placido ring type to the cornea 2 of the eye 1, and the light detector 40 of the topographer detects the ring images reflected from the cornea 2. Then the topographer calculates the shape and the curvature of the corneal epithelial surface by analyzing the size and the shape of the reflected ring images. In order to measure the thickness of the cornea 2 with a conventional topographer, a slit-shaped light is additionally projected from a light source (Not shown) to the cornea 2 with a slanting angle, and a detector (Not shown) detects the slit-shaped light reflected at the endothelial surface of the cornea 2 to obtain the shape of the endothelial surface of the cornea 2. The thickness of the cornea 2 can be obtained from the information relating the shapes of epithelial surface and the shape of the endothelial surface of the cornea 2.
However, in the conventional topographer, it is structurally impossible to obtain the ring images reflected at the vertex of cornea 2. Namely, the ring images from the measuring light source 32 are reflected only at the peripheral portion of the cornea 2 rather than the central portion thereof. Therefore, the shape and the thickness of the corneal vertex portion (i.e., the central portion) having a convex shape of a diameter of about 1 mm cannot be measured with the conventional topographer. Due to this restriction, the operation on cornea, such as the LASIK operation is performed without accurate information on the shape and thickness of the central portion of the cornea 2. In addition, the conventional topographer consists of only various lenses and mirrors. Thus, the topographer should be assembled very precisely, and the accuracy of the measurement is deteriorated as the topographer is used.
FIG. 2 is a drawing for illustrating the optical system of the conventional refractometer having an additional function of measuring a corneal curvature. As shown in FIG. 2, the conventional refractometer comprises a chart image generating part 20 for fixating the gaze of the examined eye 1 and performing a fogging process to blur a focus of the eye 1; a measuring light source 10 for projecting a measuring light to the retina 3 of the eye 1 for measuring a refractive power of eye 1; a measuring light detecting part 12 for receiving the measuring light reflected from the retina 3 of the eye 1; and a light detector 40 for detecting the chart image reflected from the retina 3 of the eye 1 in order to perform the fogging process. The refractometer shown in FIG. 2 further includes mire ring 5 for radiating a ring-shaped measuring light to measure the corneal curvature.
In operation, a visible light produced by the lamp 22 of the chart image generating part 20 passes through the chart 24. The chart image formed at the chart 24 is projected to form the chart image on the retina 3 of the eye 1 through the first relay lens 25, a reflecting mirror 26, the second and the third relay lens 27,28, and the first and the second beam splitter mirror 35, 36. Meanwhile, an image of the eye 1 produced by an illuminating light (not shown) is projected to the light detector 40 through the first and the second beam splitter mirror 35, 36, the third relay lens 28, and the fourth relay lens 37. The operator aligns the optical axis of the ophthalmic apparatus and eye 1 by detecting the image of the eye 1 formed on the light detector 40. Then the “fogging operation” is carried out to relax the eye 1 to be examined. In detail, the operator adjusts the position of the first relay lens 25 so that the eye 1 is focused to the chart image. Then the first relay lens 25 is controlled to move a position at which the eye 1 cannot be focused to the chart image. This process makes the intraocular lens of the eye 1 to be relaxed, and the refractive power is measured under this relaxing condition
After completing the fogging operation, the refractive power measuring light is projected from a measuring light source 10. The measuring light radiated from a measuring light source 10 is condensed by the fifth relay lens 11, passes through the center of a hole mirror 19, and projected to the retina 3 of the eye 1 through an objective lens 34 and the first beam splitter mirror 35. The measuring light reflected from the retina 3 passes through the first beam splitter mirror 35, is reflected by a hole mirror 19, is separated by a six-holes plate 13 into six measuring lights. The six measuring lights are condensed by the sixth relay lens 14, and projected to a light detector 15. The propagation directions of the six measuring lights separated by the six-holes plate 13 are changed according to the refractive power of the eye 1 to be examined. Thus, the refractive power of the eye 1 can be calculated by analyzing the images of the six measuring lights formed on the light detector 15.
In measuring the corneal curvature with the ophthalmic apparatus shown in FIG. 2, a ring-shaped light is projected to the cornea 2 of the eye 1 from a mire ring 5. The ring-shaped light reflected from the cornea 2 of the eye 1 passes through the first and the second beam splitter mirrors 35,36 and is projected to the light detector 40. The corneal curvature can be measured from the degree of the distortion of the ring-shaped light projected to the light detector 40.
In the above described conventional ophthalmic apparatus, all of the optical components are fixed to an optical base, and the signal reflected from the retina 2 overlaps with the signal reflected from the surface of the optical components disposed between the light detector 15 and the retina 2, which may produce data-distortions. If a light radiated from a light source is projected to the eye with a predetermined angle with respect to an optical axis of the eye to be examined, such distortion can be avoided. However, such ophthalmic apparatus is very complex in its configuration, and should be controlled very precisely. In addition, the images of the six measuring lights formed on the light detector 15 are significantly varied in their sizes according to the refractive power of the eye 1, which may increase the measurement error.